One  annoying  factor is the cross-section
for Air target.  For the proton target, we know
experimental value and some fit extensible to
higher energies,  but it must be converted
to Air target value.   We use normally
simplified Glauber calculation; for small
cross-section like for neutrino,  cross-section
for mass number A is S(A)=AS(p).    
Photo-hadron cross-section is  small 
but not so small as neutrino.  Some says,
S(air) = A^0.91 S(p). Simplified Glauber
calculation  in Cosmos gives higher values
than this.  Some other says little bit 
different treatment and the current Cosmos
employs that value and is very close to A^0.91
dependence.   However, recently I found a book
that says  Glauber calculation for A gives
S(A)= A^(2/3)S(p)  (Cosmic ray astrophysics:
by Reinhard Schlickeiser).

